The Sealing Principle and Factors of Flange Gaskets

Factors Affecting Sealing of Flange Gaskets

Through the pre-tightening force of the bolt, the sufficient pressure is generated between the gasket and the flange sealing surface. The deformation of the gasket surface is sufficient to fill the micro-unevenness of the flange sealing surface, thereby achieving the purpose of sealing. Today, we will explain the sealing principle of flanges and the factors that affect sealing.

The minimum unit pressure that acts on the fange gaskets to achieve the above purpose is called the specific pressure. When the pipeline connected to the flange reaches the working pressure, the axial force of the internal pressure causes the two flanges to have a tendency to separate, while the bolts have a tendency to elongate, and the pressure on the gasket will decrease. When the pressure acting on the effective cross-section of the gasket drops to a certain critical value, the seal can still be maintained. At this point, the remaining pressure on the gasket is the effective tightening force of the gasket. When the pressure on the gasket is less than the effective tightening force, the flange may leak or even blow away the gasket, so the effective tightening force of the gasket must be greater than the working pressure of the pipeline. The distance between the two flange sealing surfaces is greater than the initial working state. At this point, the tightness of the gasket and flange sealing surface is ensured by the rebound force of the gasket.

It can be said that in the early stages of sealing, the plastic deformation of the gasket surface plays a decisive role in the micro non-uniformity of the sealing surface of the filling flange. In the operating state of flange sealing, the elastic recovery inside the gasket plays a dominant role.

The main factors affecting sealing are as follows.
(1) The operating conditions are the pressure, temperature, and physicochemical properties of the medium. There are many low-pressure flanges in petrochemical plants, and the influence of pressure or medium on flange leakage is not the main factor. This problem is only serious when combined with temperature. When the temperature changes repeatedly, the possibility of sealing failure is higher.

(2) The influence of gasket coefficient and compression ratio on design parameters. At present, both parameters are selected by looking up the table for "steel pressure vessels", but even if the gasket coefficient and compression ratio are of the same material, they are still related to factors such as gasket width, preloading pressure, medium performance, flange sealing surface width, and roughness. Therefore, there is no authoritative standard data to choose these two coefficients.

Sealing principle of flange gaskets

The flange gasket and fastening bolts form a sealing system. There is always roughness on the contact surface of a pair of flanges. It is not possible to achieve sealing without gaskets and relying solely on bolt clamping.  As a sealing element, the gasket plays an important role in ensuring sealing. 

The sealing mechanism of flange gasket static sealing can be divided into the following two situations.

1. Initial sealing
The initial sealing can be completed when assembling the flange gasket. The basic condition for tightening the bolts and compressing the gasket to generate initial sealing is to generate sufficient pressure between the sealing surfaces, that is, the gasket pre-tightening stress causes the gasket material to undergo elastic compression deformation or elastic-plastic compression deformation after compression, which can fill the rough and uneven sealing surface of the flange, block the interface leakage channel to meet the initial sealing requirements. The pre-tightening stress of the gasket depends on the compression performance of the gasket material. All sealing pressure comes from the initial tightening force of the bolts, and the flange should have sufficient strength and sufficient stiffness to prevent deflection or warping deformation.

2. Work sealing
When subjected to the pressure of the working medium, the pre-tightened flange gasket bolt three-component system will force the two sealing surfaces of a pair of flanges to undergo relative separation. At this time, the gasket should have sufficient rebound deformation to compensate for this separation. Not only that, the gasket also requires the ability to maintain sufficient residual gasket stress (compressive stress) at the sealing interface to ensure sealing of the fluid medium.

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